this is a vanilla bug they didn't always wear it to begin with but in the vanilla game there was only 3 types of enclave power Armour and they were went :advanced power Armour<Tesla Armour<hellfire Armour

however the mod added a 4th Armour type and it seems to be lots more common than the other types to the point where every hellfire trooper i have seen has had it

It's easy enough to fix... just go into the template for the Hellfire trooper, change the tier 4 enclave armor listing to the specific Hellfire armor. That way they always wear it, and the armor isn't selected from the list.

I might be wrong, but with the GotY edition of vanilla Fo3, this issue didn't really seem to appear and if it did it was so rare as to be barely noticeable. I can confirm having the same issue though. Still, this could just be incomplete; I cheated in a set of Hellfire Armor and I think it had over 40 DT!